Looking at this question, I wonder what good and useful web services are out there. For example the LaTeX symbol classifier looks interesting. You can draw a symbol with your mouse and get back the LaTeX name of that symbol (the software analyzes your drawing). There are many others out there. Which one do you use regularly? Please provide a link and a short description.

As it stands, I feel that this question is too broad and imprecise. What do you mean by "web service"? I can download detexify and run it on my own machine, or I can install a TeX-server via apache, so what distinguishes a "web service" from a non "web service"?
–
Loop SpaceSep 23 '10 at 8:48

I mean "a web page that you can visit". So you only need a browser to access something that is useful for your (daily) TeX/LaTeX/ConTeXt work. I am not talking about wiki pages or blogs or similar. The MathTran project is such an example. There must be many more out there.
–
topskipSep 23 '10 at 8:54

mathurl You input a formula in a box and get an instantaneous render (as png) which you can then shorten and embed. They support amsmath and have shortcuts on the side for common symbols, operators, font styles etc. Great for tossing in an equation or two in an email or forum post on the fly.

This web app seems fairly new and does a good job of recognizing hand-written equations. It works on iPhone or iPad so you can draw equations better than with a mouse. In return, it provides an image of the equation and the LaTeX or MathML code: http://webdemo.visionobjects.com/equation.html

My go-to-web favourite is this nice little Online LaTeX Equation Editor for making the business of assembling complex math LaTeX code a complete non-brainer. The interface is very intuitive and the online preview makes it easy to immediately see what's going right and wrong. Its preview function extends its usefulness well beyond the TeX domain. You can select a good range of display options (typeface, font size, colour, transparency, dpi, etc.), then download its output in one of a number of formats (gif, png, pdf, etc.) for those times you or non-TeX others might want to embed LaTeX-typeset formulae into non-TeX applications.

write-math.com

This service allows classification by drawing (even works on mobile devices!) and by text search:

Disclaimer: I am the author of the service.

Some information

The handwriting recognition toolkit (hwrt) is one possibility to classify you recordings. There are still many rough edges and the software gets updated on a daily basis (11.06.2015). The user interface is in a browser and looks like this:

The installation is explained in the documentation. If you have trouble or have an idea how to improve it, just leave a comment or write an email (info@martin-thoma.de).